In a porous body comprising valve action metal such as tantalum, niobium or the like, a dielectric layer comprising oxide with a controllable thickness can be formed in the surface of the porous body by using an anodic oxidation, and such porous bodies have been widely employed as anode elements for electrolytic capacitors by making use of the wide surface area of the porous body. Particularly as tantalum has high resistance to heat and corrosion, it has been employed as a sintered body for filament material, chemical equipment parts, artificial bone and the like, and its use as an electrolytic capacitor is overwhelming.
Technology in miniaturization of surface-mounted devices have progressed rapidly in recent years, and technologies for mounting to component boards in electronic devices such as cellular telephones, personal computers and digital cameras have been highly integrated. Under this background, various research efforts have also been conducted in the field of capacitor elements, which are electronic parts, catching up with the demand for a smaller, thinner, larger capacity product.
Among capacitor elements that are typically employed at the present time, in particular tantalum electrolytic capacitors have the property of enabling a large capacity in a small size, and have been greatly researched with the goal of further decreasing size and thickness.
Examples of materials having the same properties as tantalum metal, i.e., so-called valve action metals, include various metal materials like aluminum, niobium and titanium, however, tantalum metal is highly demanded for its heat resistance and forming capability of a dielectric layer.
As for a method for producing an electrolytic capacitor employing the aforementioned valve action metal powder such as tantalum for example, typically, the tantalum is used as an anode metal powder, and the tantalum metal powder and resin which functions as binder are filled in a metal die, and are subjected to pressure processing and chipping, to form a molding for anode element.
A component (typically a tantalum lead wire) which becomes the anode terminal is provided to the molding for an anode element. This lead wire is typically inserted within the tantalum metal powder set in the metal die, and is fixed in place by pressure molding of the tantalum metal powder.
An anode element obtained through the above steps then goes through a process to vaporize and remove the unnecessary resin therein by subjecting the element to high temperature heating treatment in a vacuum.
Through this process, in which the resin present in the tantalum metal powder is vaporized and removed and the tantalum metal particles are fused at their points of contact, a tantalum electrolytic capacitor anode element with a porous body is obtained.
A tantalum electrolytic capacitor anode element obtained in this way is dipped in a electrolytic bath, a prescribed DC voltage is applied, and a chemical conversion treatment is carried out, to form a dielectric layer comprising tantalum oxide on the surface of the tantalum metal powder, thereafter, a solid electrolyte coating of manganese dioxide or functional polymer coating is formed on the dielectric layer.
Next, after a process of forming a cathode layer using carbon and silver paste is carried out and resin outer sheathing is provided, to obtain the final tantalum electrolytic capacitor.
In response to the demands for smaller and thinner electrolytic capacitors, research has been proceeding to further decrease the size and thickness of the dimensions of the capacitor. The capacitor is made thinner, for example, so as to be embedded in the board=or to be laminated, so that it is possible to realize low Equivalent Series Resistance (ESR) and to improve high frequency properties. However, in conventional methods using a die, it is difficult to make a thin-type electrolytic capacitor anode element with a thickness of 0.5 mm or less efficiently, and moreover, it has also been difficult to form a large type electrolytic capacitor anode element even if the thickness is around 1 mm.
As for a method for making the aforementioned anode element thinner, Japanese Patent Application, First Publication No. Sho. 56-83022 discloses a method for producing an electrolytic capacitor anode element in which valve action metal powder, binder including a thermoplastic resin, and an organic solvent are mixed together to form a paste; and by using the paste, a sheet is formed (i.e., a molding in the form of a thin film), and lead wire is connected to the sheet; and a debinder treatment is carried out, and sintering is performed to obtain the desired product.
However, when forming a sheet by using a paste comprising a mixture of valve action metal powder, binder resin, and an organic solvent as described above, there were the following problems to be solved.
In particular, when an electrolytic capacitor anode element is made by sintering a molding for this purpose, it is preferable that the quantity of binder resin be limited to as small as possible in order to ensure good capacitor properties such as low residual carbon and small leakage current, however, the reduction of the blending amount of binder resin causes to the decrease of the mechanical strength of the sheet (molding) and to difficult handling. In some cases, yield may decline due to, for example, damage to a portion of the molding during the separation of molding from the substratum or after the separation of molding from the substratum, in the production process.
In other words, when the blending amount of the valve action metal powder is increased, the adhesive effect provided by the binder resin decreases, and in particular, when the element is made thinner, then the molding is more fragile, accordingly, the binder resin is required to be present at a specific proportional amount or greater.
On the other hand, if the blended binder resin is sufficient to ensure strength, the residual carbon increases as described above, and as a result, causes such problems as degradation in the electric properties of the capacitor anode element.